This invention relates to an improved method and apparatus for bonding thermoplastic materials to one another.
Techniques for bonding thermoplastic materials to one another have been known for quite some time. Examples of such bonding techniques are described in Welding of Plastics, Neumann and Bockhoff, Reinhold Publishing Co., 1959, and include hot plate and friction welding. By either of these techniques, the edges of the plastic materials to be bonded are heated to bring the plastic at the edges to its fusion temperature. As soon as the edges are sufficiently heat-softened, they are quickly joined together under pressure until the melted or softened edges have cooled sufficiently to form a strong joint. During the welding operation, the pressure between the two softened edges of the thermoplastic materials should be sufficient to force out air bubbles and to bring the entire edge surfaces into intimate contact. The resulting pressure on the softened edges as they are joined together results in the formation of a rounded bead along the junction of the two thermoplastic materials. In the past after the bonded or welded edges cooled, the rounded bead was removed by sanding in an area about the juncture of the bonded edges or by cutting away the bead. This was followed by a polishing step.
In many applications, however, the integrity, reliability and durability of the weld or bond is of critical importance. As one example, when thermoplastic pipes are bonded to one another by means of a hot plate weld, it is very important that the weld have the required strength and durability in order to serve the purpose of conveying fluids under varying temperatures and pressures in an environment which may be subject to substantial vibrations. As a second example, some battery jars are formed by hot plate welding techniques. These battery jars contain a liquid electrolyte and support a series of heavy electrodes. When placed in situ, the battery jars are subjected to vibration and occasional shock impulse forces, and accordingly, the welds must be of substantial strength and durability to remain functional over a long period of time.
In order to test the integrity and reliability of these welds a number of techniques have been developed. One method is to establish a very high electromagnetic field across the weld to determine whether dielectric breakdown occurs. If there are minute pores and/or cracks in the weld, the dielectric strength of the weld will be reduced and upon establishing the electromagnetic field across the weld, a spark will be generated.
Another technique for testing the integrity and reliability of welds is to generate a mechanical impulse force against the weld to determine its resistance to fracture. In the battery jar industry this is accomplished by dropping a weighted dart from a preset distance onto the weld to generate a very high point pressure differential across the weld. Of course, other impact techniques can be used depending upon the design requirements of the finished product. These techniques for measuring the reliability and strength of welds have proven useful in many applications where the integrity of a weld joint is of critical importance.
Using these and other known testing techniques, it has been found that the formation of hot plate welds by the simple heating of the edges of the thermoplastic materials to be joined and then forcing the edges against one another to form the weld results in decreased tensile strength of the material at the weld junction; that is the tensile strength of the material at the weld junction can be 85 percent of the tensile strength of parent material and lower. In addition, the dielectric test failure rate resulting from generating a large electromagnetic field across the weld increases as much as 100 times over the dielectric test failure rate of the parent material. Further, the impact strength of such welds when tested by dropping a dart onto the weld was found to be reduced substantially over that of the parent material and in addition varied substantially at different points along the welds and from one weld to the next to thereby reduce the overall reliability of the weld. Further, the bending strength, particularly the flexural deflection, of the weld about the axis of the weld was found to be reduced substantially.
It is therefore an object of this invention to provide an improved method of bonding thermoplastic materials to one another to improve the strength and reliability of the bond.
It is another object of this invention to provide an improved apparatus for bonding thermoplastic materials to one another.